text: Iodine is known to be crucial for life in many unicellular organisms Phosphorylated tyrosines created with tyrosine kinases are fundamental signalling molecules in all animals and in Choanoflagellates and may be linked to the usage of tyrosine iodine compounds for similar roles. Crockford proposes that iodine was originally used in protecting cell membranes from oxidative damage in photosynthesis and later moved into cytoplasm and became involved with balancing cytoplasmic composition of ions, and later the non enzymatic synthesis of tyrosine in early life. 
  It is common across all domains of life and uses tyrosine bonded to iodine.
  Plants, insects, zooplankton and algae store iodine as mono-iodotyrosine (MIT), di-iodotyrosine (DIT), iodocarbons, or iodoproteins.
  Many plants use thyroid like hormones for regulating growth.
  Gut-inhabiting bacteria use iodine from host thyroid hormone.
  Thyroid-like hormones may be linked to the development of multicellularity. Iodotyrosines are highly reactive with other molecules which may have made them important cell signalling molecules early in evolutionary history. They form spontaneously without need for enzymatic catalysts which may have contributed to their early adoption by organisms, although enzymes make the yields significantly higher.
  The ease of reaction with water may explain why iodine is so common across cell signalling in all domains of life.
  Many photosynthetic microbes are able to reduce inorganic iodate to iodide in their cell walls but much of it gets released into the environment rather than cytoplasm in compounds such as methyl iodide. Many sulfate-reducing microorganisms and Iron-oxidizing bacteria also reduce iodate to iodide as well as many facultative anaerobic organisms suggesting this may be ancestral among anaerobic organisms.
  Kelp store large quantities of iodide primarily as iodotyrosines for unknown reasons.
  Molecular iodine (I) is toxic to most single-celled organisms by disrupting the cell membrane however Alphaproteobacteria and Choanoflagellates are resistant. Organisms such as Escherichia coli are killed by molecular iodine but require iodine from host thyroid hormone, indicating that not all organisms that need iodine are resistant to the toxic effects of pure iodine.